Porcupines' quills, or spines, take on various forms, depending on the species, but all are modified hairs coated with thick plates of keratin, and they are embedded in the skin musculature. Old World porcupines (Hystricidae) have quills embedded in clusters, whereas in New World porcupines (Erethizontidae), single quills are interspersed with bristles, underfur and hair.
Quills are released by contact with them, or they may drop out when the porcupine shakes its body. New quills grow to replace lost ones. From ancient times, it was believed that porcupines could throw their quills at an enemy, but this has long been refuted.
